I've owned a No Mar for close to 10 years, it LOOKS easy on YouTube and in person, I even visited the shop on STL but and this is a big BUT, trust me, until you learn the technique perfectly and use shortcuts I mentioned most attempts end in frustration, failure and No Mar machines for sale.
Those GT tires are extra stuff, and when installed on certain rims, like BMW rims with a shallow drop center, well, they aren't the best choice to learn on. Can be done, I've done many on my GT and RT, just challenging.
Don't mean to discourage, once you find your way it's great to have and I'd never go back to paying dealer tire prices and having someone else damage my rims, I've changed well over 100 tires and still it sucks from time to time.